HttpWebRequest httpWebRequest = (HttpWebRequest)WebRequest.Create("......");
httpWebRequest.Method = "POST";
httpWebRequest.ContentType = "application/x-www-form-urlencoded";
using (StreamWriter sw = new StreamWriter(httpWebRequest.GetRequestStream()))
    sw.Write(Encoding.GetEncoding(1251).GetBytes("id=152&PID=2415"));
using (HttpWebResponse httpWebResponse = (HttpWebResponse)httpWebRequest.GetResponse())
using (Stream stream = httpWebResponse.GetResponseStream())
using (StreamReader reader = new StreamReader(stream,Encoding.GetEncoding(1251)))
{
    if (httpWebResponse.StatusCode == HttpStatusCode.OK)
        return reader.ReadToEnd();
}